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Overview of BAX and Apoptosis
By definition, apoptosis is programmed cell death, or suicide. To induce cell death among cancer cells is a critical component to cancer treatment. Specifically, the Bcl-2 protein family members have been found to have a prominent role in apoptosis, yet researchers have not been able to deduce more information on their mechanisms. Bcl-2 proteins occupy the mitochondrial outer membrane permeabilization, and are recognized either as pro-apoptotic (Bax, BAD, Bak, and Bok) or anti-apoptotic (Bcl-2 proper, Bcl-xL, and Bcl-w). Pro-apoptotic Bcl-2 proteins are death-promoting members while anti-apoptotic members are death-inhibiting structures. Currently, researchers have identified 25 genes in the Bcl-2 family. [4]

One particular Bcl-2 protein is the Bax protein, one of the death-promoting members. To date, researchers have found great trouble deciphering the nature of the Bax protein in the cell. Past studies have overexpressed the Bax protein in in vitro and in vivo studies to discover more information on its function. However, some of the criticism from such methods is that these conditions are not indicative of the normal activity of Bax. Either way, there have been mixed results through these types of studies. [4]

Structure
Bax proteins contains 9 alpha helices while alpha-1 through alpha-8 are similar to that of Bcl-xL. The C-terminal alpha-9 helix occupies the hydrophobic pocket, which arbitrates the heterodimer formation and bioactivity of differing members of the Bcl-2 family. Researchers from the 2000 article by Suzuki et al. determined that the Bax structure indicates that the orientation of the alpha-9 helix offers concurrent control over its mitochondrial targeting and dimer formation. [3]

In another article by Gavathiotis et al. (2008), the authors discovered through NMR analysis that the BIM stabilized alpha-helix of Bcl-2 (SAHB) domain binds Bax at an interaction site different from the antiapoptotic proteins. The Bax binding site was also characterized by lysine at position 21 (K21), glutamine at positions 28 and 32 (Q28, Q32), arginine at position 134 (R134), and glutamic acid at position 131 (E131). [4]



Chipuk et al. (2004) Study
In the Chipuk et al. study from 2004, the authors determined that the Bax protein is directly activated by the tumor suppressor gene p53, which in turn mediates mitochondrial membrane permeabilization and apoptosis. Furthermore, the researchers discovered that cytosolic localization of endogenous wild-type or trans-activation deficient p53 was essential and adequate for apoptosis. Lastly, the transcription-independent activation of Bax by p53 happened with comparable kinetics and concentrations to that of activated Bid. [1]

Zhang et al. (2000) Study
In the 2000 study by Lin Zhang, Jian Yu, Ben Ho Park, Kenneth W. Kinzler, and Bert Vogelstein titled “Role of Bax in the Apoptotic Response to Anticancer Agents,” the experimenters evaluated the role of the Bax protein in drug-induced apoptosis in human colorectal cancer cells. The review described several experiments that all attempted to elucidate the mystery behind the Bax protein. [4] For the first experiment, the researchers used the fact that chemotherapeutic agents usually target epithelial cells to clarify the role of Bax in drug-induced apoptosis in epithelial cells through the creation and analysis of isogenic derivatives that vary only in the presence or absence of the Bax gene. The results from this experiment showed that 2% of HCT116 (or epithelial parental cells) have two intact Bax alleles (+/+), 94% had one intact allele (+/-), and 4% had two mutant alleles (-/-). [4]

In another experiment, the experiments presented that hypothesis that if Bax deficiency greatly affects the sensitivity to non-steroidal anti-inflammatory drugs (NSAIDs), which were previously test in a different experiment, then the parental cell populations treated with NSAIDs result in a mutated Bax cell population. After recovering grown Bax cells from an in vitro study with the NSAID indomethacin. The results showed that 70% of the Bax cells had insertions or deletions in the G8 tracts of both Bax alleles while 4% had mutations in the parental population. [4]

Ultimately, in regards to the clinical implications from this study, the results suggest that colorectal tumors may easily develop resistance to NSAIDs through an inherent instability in the mononucleotide tract (at nucleotides 114 to 121 or codons 38 to 41 ) in BAX, but more importantly, a combination of chemopreventive drugs must be considered as a leading cancer treatment. [4]

Reference
[1] Chipuk, J.E., Kuwana, T., Bouchier-Hayes, L., Droin, N.M., Newmeyer, D.D., Schuler, M., and Green, D.R. Direct activation of Bax by p53 mediates mitochondrial membrane permeabilization and apoptosis. Science 303: p. 1010-1014.

[2] Gavathiotis, E., Suzuki M., Davis, M.L., Pitter, K., Bird, G.H., Katz, S.G., Tu, H.C., Kim, H., Cheng, E.H., Tjandra, N., Walensky, L.D. BAX activation is initiated at a novel interaction site. Nature 455: p.1076-1081.

[3] "OMIM - BCL2-ASSOCIATED X PROTEIN; BAX." NCBI HomePage. Web. 01 Nov. 2009. .

[4] Zhang, L., Jian, Y., Park, B.H., Kinzler, K.W., and Vogelstein, B. Role of Bax in the apoptotic response to anticancer agents. (2000). Science 290 (5493): 989-992.